Percutaneous transluminal coronary angioplasty (PTCA) is widely accepted as an effective treatment of blockages in the coronary arteries. Blockages (stenoses) may occur from cholesterol precipitation on the coronary wall which may be in any stage from initial deposit through aged lesions. Coronary arteries can also become blocked due to formation of thrombus.
The most widely used percutaneous coronary angioplasty makes use of a dilatation balloon catheter. Typically, a guide catheter is employed to facilitate the introduction of the balloon catheter into the patient's vascular system. That is, the guide catheter is first introduced, and then the dilatation balloon catheter, with the balloon at its distal end in a deflated condition, is pushed through the lumen of the guide catheter until the balloon exits the distal end of the guide catheter. A guide wire coupled to the balloon catheter may be used to assist in pushing the balloon catheter through the guide catheter and in steering the distal end of the balloon catheter to the region of the stenosis or lesion. A radiographic contrast fluid is then fed under pressure through an inflation lumen of the dilatation catheter to the balloon, which causes the balloon to expand outward, thereby opening the stenosis.
In some angioplasty procedures, it is necessary to exchange one dilatation catheter for another. For this reason, dilatation balloon catheters are typically designed so that they are capable of being advanced and retracted along the guide wire.
One important characteristic of dilatation balloon catheters used for angioplasty is profile, i.e., the outer shape and diameter of a catheter's distal end portion when in a deflated condition. Considerable effort has been spent in developing low-profile dilatation balloon catheters by minimizing the dimensions of the core or inner tube which extends through the balloon to its distal end, and by reducing wall thickness, to the extent possible, of the balloon itself.
The outer diameter of the deflated distal end portion of a dilatation balloon catheter affects the ease and ability of the dilatation catheter to pass through the lumen of the guide catheter, through the coronary arteries, and across tight lesions.
A complicating factor in minimizing the deflated profile of a dilatation catheter balloon is that the balloon membrane is typically not distensible, i.e., it does not stretch or contract in response to changes in internal pressure. Thus, the balloon membrane has a constant surface area regardless of whether the balloon is inflated or deflated. Therefore, in order to reduce the outer diameter of the balloon in its deflated condition, it is common to fold the balloon flat, so that two wings or flaps are formed. These two wings are then brought together in some fashion, as by folding or wrapping, so as to reduce the overall diameter of the deflated balloon. Often, some sort of protective sleeve or sheath is disposed around the folded or wrapped balloon to protect the balloon from contamination or damage prior to its use.
In actual use, when inflation fluid is applied to the folded balloon, it causes the flaps to unwrap so that the balloon can inflate to its full inflated state.
Thus, it is desirable to minimize profile of any angioplasty balloon, or, in other words, to provide any inflated balloon diameter with the smallest possible profile. One practical effect is that the two flaps formed when the balloon is deflated and prepared for wrapping (during balloon protector installation) become very large relative to the core or inner tube of the catheter. The result is that it is difficult to get these two large flaps to fold together and squeeze out all of the space between them when folded, without damaging the catheter during balloon protector installation.
Various methods and balloon configurations have been proposed in the prior art for providing a dilatation balloon catheter having the lowest profile as possible when deflated and the largest possible diameter when inflated. One approach, which is suggested, for example, in U.S. Pat. No. 5,087,246 to Smith and in U.S. Pat. No. 5,147,302 to Euteneuer et al., is to provide a dilatation balloon which in deflated condition has more than two flaps or wings (for example, three wings) such that when the flaps or wings are wrapped circumferentially, the distance that each flap extends around the catheter is reduced compared with the conventional balloon configuration having only two flaps. The ease with which such flaps fold is also enhanced when their number is increased, such that when the balloon is deflated and withdrawn through the guide catheter following a procedure, the balloon more readily returns to its wrapped condition. The result is a reduced deflated profile given the same inflated diameter.
Another important characteristic of dilatation balloon catheters is the ease with which they can be advanced and retracted along a guide wire for the purposes of exchanging one catheter for another during an angioplasty procedure. In early dilatation balloon catheters, sometimes called "over-the-wire" catheters, the guide wire passed through the catheter along the catheter's entire length. U.S. Pat. Nos. 5,040,548 and 5,061,273 to Paul G. Yock, on the other hand, propose so-called "rapid-exchange" catheters having a guide wire lumen along a distal section of the catheter only. In the event that the dilatation catheter must be removed for the purposes of exchange, it is withdrawn along the guide wire, the guide wire being left in place in the vascular system. The distal end of a different dilatation catheter is then threaded onto the proximal end of the guide wire and advanced forward along the guide wire. When the guide wire is threaded only through a distal section of the dilatation catheter, there may be some reduction in friction between the catheter and the guide wire, and the length of the guide wire may also be reduced.
A similar approach is proposed in U.S. Pat. No. 4,762,129 to Bonzel (see also Reexamination Certificate B1 4,762,129). In the Bonzel patent, a "rapid-exchange" catheter is described in which a guide wire runs parallel with the inflation lumen of the catheter, and is threaded only through the distal end of the catheter, in the region of the balloon itself.
Catheters in accordance with the designs proposed in the above-referenced Yock '548 and '273 and Bonzel '129 patents have been commercially-available, and the construction of such catheters is well-known in the art.
While the above-described references may represent some improvement in field of balloon dilatation catheters, the inventor believes that there is an ongoing need for improvements in catheter design and preparation techniques, such that low (deflated) profile and large inflated balloon diameters may be achieved without sacrificing other characteristics. Additionally, it is believed that there is also an ongoing need for improvements which facilitate the rapid and efficient exchange of dilatation catheters during a procedure.